Compositions of epichlorohydrin rubber and polyester

ABSTRACT

Compositions are described comprising blends of cured epichlorohydrin rubber and crystalline polyester.

This invention relates to polymer blend compositions and, moreparticularly, to thermoplastic compositions comprising blends ofcrystalline polyester and cured epichlorohydrin rubber.

BACKGROUND OF THE INVENTION

Thermoplastics are compositions which can be molded or otherwise shapedand reprocessed at temperatures above their melting or softening points.Thermoplastic elastomers (elastoplastics) are materials which exhibitboth thermoplastic and elastomeric properties, i.e., the materialsprocess as thermoplastics but have physical properties like elastomers.Shaped articles may be formed from thermoplastic elastomers byextrusion, injection molding or compression molding without thetime-consuming cure step required with conventional vulcanizates.Elimination of the time required to effect vulcanization providessignificant manufacturing advantages. Further, thermoplastic elastomerscan be reprocessed without the need for reclaiming, and, in addition,many thermoplastics can be thermally welded.

Moldable thermoplastic compositions of polyester and uncuredepichlorohydrin rubber are known (Koleske, et al, U.S. Pat. No.3,781,381). Moldable elastoplastic compositions of polyester and curedhigh unsaturation diene rubbers containing high proportions of rubberare known (A. Y. Coran and R. Patel, U.S. Pat. No. 4,141,863).

SUMMARY OF THE INVENTION

It has been discovered that compositions comprising blends ofthermoplastic crystalline polyester and cured epichlorohydrin rubberexhibit a valuable combination of properties. Generally, compositionscomprising about 5-75 parts by weight of polyester and, correspondingly,95-25 parts by weight of cured epichlorohydrin rubber are moldablethermoplastic compositions exhibiting improved strength, greaterelongation, greater toughness or impact resistance, or improved truestress at break. A preferred composition comprises a blend of about20-60 parts by weight of polyester, and correspondingly, about 80-40parts by weight of cured epichlorohydrin rubber. Cross-linking therubber increases the tensile strength and improves tension set andtoughness of the composition. In addition, cross-linking the rubberimproves the solvent resistance and the high temperature properties ofthe blend. The properties of the composition improve as the extent ofcross-linking of the rubber increases. In preferred compositions, theepichlorohydrin rubber is cured to the extent that no more than 20weight percent, preferably no more than 10 weight percent of the rubberis extractable in a solvent in which uncured epichlorohydrin rubber isessentially completely soluble. Compositions comprising 60 or more partsby weight of polyester per 100 parts by weight of both the polyester andcured epichlorohydrin rubber are thermoplastic compositions exhibitingimproved impact resistance. Compositions comprising 40 or more parts byweight of cured epichlorohydrin per 100 parts by weight of rubber andpolyester are elastomeric. When compositions containing high proportionsof rubber are statically cured, such as in molds, thermoset compositionsare obtained; whereas, if such compositions are dynamically cured, i.e.,if the blend is masticated while the rubber is cured, elastoplasticcompositions are obtained which compositions exhibit elastomericproperties, and yet and processable as thermoplastics.

The relative proportions of polyester and cured epichlorohydrin rubberof the elastoplastic compositions of the invention are not subject toabsolute delineation because the limits vary, due to a number of factorsincluding type, molecular weight, or molecular weight distribution ofthe polyester or rubber, the type of rubber, and type and amount ofcurative used to cure the rubber. The amount of polyester must besufficient to impart thermoplasticity to the compositions, and theamount of cured epichlorohydrin rubber must be sufficient to impartrubberlike elasticity to the composition. The term "rubberlikeelasticity" means for the composition to have a tension set value ofabout 50% or less. The range of proportions for which the composition iselastoplastic may be ascertained in a few simple experiments by thoseskilled in the art by following the teachings herein. Generally,elastoplastic compositions of the invention comprise blends of about10-65 parts by weight of polyester and, correspondingly, about 90-35parts by weight of cured epichlorohydrin rubber per 100 total parts byweight of polyester and rubber. More preferred compositions comprise25-60 parts by weight of polyester and, correspondingly, 75-40 parts byweight of epichlorohydrin rubber. Blends containing lower proportions ofpolyester generally exhibit better tension set, whereas, blendscontaining higher proportions of polyester exhibit higher stress-strainproperties, including true stress at break, TSB.

It is important for thermoplasticity that the cured rubber is present inthe form of small dispersed particles, otherwise, the composition willeither be weak or not processable as a thermoplastic. If the rubber isnot dispersed and forms a somewhat continuous phase throughout theblend, a thermoset composition not processable as a thermoplastic may beobtained. The dispersed rubber particles must be small enough tomaintain strength and thermoplasticity of the composition. If theparticles are too large, weak, low strength blends are obtained.Compositions containing still larger particles may not be processable asthermoplastics. Accordingly, it is understood that in elastoplasticcompositions of the invention, the particle size is small enough tomaintain high strength and thermoplasticity. Generally, the cured rubberparticles are of a size of about 50 microns number average or less. Thesmaller the particle size, the better the properties, including strengthand processability. Preferably, the particle size is about 10 micronsnumber average or less. The dynamic curing process, when carried outproperly, can give cured rubber particles within the range of about 0.1to 2 microns number average.

In order to achieve the improved compositions of the invention, it isessential that the rubber is cured with enough rubber curative so thatthe rubber is cured sufficiently to give a composition having asubstantially greater ultimate elongation than a corresponding blendcontaining uncured rubber. Preferably, sufficient rubber curative isused to also give a significant increase in tensile strength. Inpreferred composition, the extent of cure of the rubberis such that thetrue stress at break, TSB, is at least two times the TSB of the similarblend in which the rubber is uncured.

Elastoplastic compositions of the invention are preferably prepared by adynamic curing process which comprises masticating a mixture of meltedpolyester, epichlorohydrin rubber, and curative at a curing temperatureuntil curing is complete. Conventional rubber masticating equipment, forexample, Banbury Mixers, Brabender Mixers, and mixing extruders, may beused to carry out the dynamic vulcanization process. The polyester andepichlorohydrin rubber typically are mixed at a temperature above thepolyester melting point, after which curative is added. Mastication atvulcanization temperature is continued until vulcanization is complete,generally within a few minutes, depending on the temperature. To obtainthermoplastic elastomeric compositions, it is desirable that mixingcontinues without interruption until vulcanization is complete. Ifappreciable curing is allowed after mixing has stopped, a thermosetunprocessable compositions may be obtained. Frequently, the cured blendis removed from the mixer and cooled, then returned and masticated againabove the melting point of the polyester. The additional masticationstep may generally improve the processability of the composition,especially when higher proportions of rubber are used. For furtherdetails concerning dynamic vulcanization and determination of the extentof cure of the rubber, refer to the Coran et al patent, supra,particuarly Columns 2-5.

The particular results obtained by the aforesaid dynamic curing processare a function of the particular rubber curing system selected.Preferably, enough curative is used to cross-link the rubber to theextent that the cross-link density of the rubber is in the order ofabout 3×10⁻⁵ to 3×10⁻⁴ moles per millileter of rubber. Of course, thecross-link density should not be too high lest the properties of thecomposition be impaired.

One embodiment of the invention consists of a thermoset compositioncomprising a blend of polyester and cured epichlorohydrin rubber whichis in the form of a continuous network rather than being in particularform. Thermoset compositions of the invention are prepared by firstblending polyester and epichlorohydrin rubber at a temperaturesufficient to melt the resin using conventional masticating equipment.The composition is then worked on a rubber mill where curatives areincorporated therein at a temperature below the activation temperatureof the curative system. The temperature can also be below the melting orsoftening temperature of the polyester resin, in which in case thepolyester would be a particulate phase especially after mill working.The curable composition is then sheeted by passage through a roll millor a shaped specimen is otherwise prepared. The sheet or shaped specimenis then cured by conventional means, typically by heating underpressure. The specimens may be cured either above or below the meltingpoint of the polyester. When a specimen is cured below the melting pointof the polyester, the physical properties of the cured specimen can bedependent upon the direction of measurement because of orientation ofpolyester particles. The degree of anisotropy of any particular specimendepends upon the proportion of polyester in the blend and the degree oforientation. Orientation of the polyester particles can be convenientlyachieved by passing a sheeted material one or more times through therolls of an even speed mill.

Methods other than dynamic vulcanization can be utilized to preparethermoplastic elastoplastic compositions of the invention. For example,epichlorohydrin rubber can be fully vulcanized in the absence of thepolyester, comminuted, and mixed with molten polyester. Provided thatthe cured rubber particles are small, well dispersed and in anappropriate concentration, thermoplastic compositions within theinvention are obtained by blending cured epichlorohydrin rubber andpolyester. In addition, provided that enough polyester is present,thermoplastic compositions may be prepared by masticating blends ofpolyester and epichlorohydrin rubber, incorporating curatives and thencuring under static conditions, such as, in a mold.

Thermoplastic compositions of the invention are all processable in aninternal mixer, to give products which, upon transferring attemperatures above the softening or crystallizing point of the polyesterto the rotating rolls of a rubber mill, form continuous sheets. Thesheets are reprocessable in the internal mixer, after reachingtemperatures above the softening or melting point of the polyester. Thematerial is again transformed to the plastic state (molten state of thepolyester) but upon passing the molten product through the rolls of therubber mill a continuous sheet gain forms. In addition, a sheet ofthermoplastic composition of this invention can be cut into pieces andcompression molded to give a single smooth sheet with complete knittingor fusion between the pieces. It is in the foregoing sense that"thermoplastic" will be herein understood. In addition, thermoplasticcompositions of the invention are further processable to the extent thatarticles may be formed therefrom by extrusion, injection molding orcalendering.

Epichlorohydrin rubbers satisfactory for the practice of the inventionare rubbery homopolymers of epichlorohydrin (ASTM type CO) and rubbercopolymers of epichlorohydrin with ethylene oxide (ASTM type ECO). Asuitable copolymer rubber may contain a small quantity of a thirdmonomer to introduce enough olefinic unsaturation into the molecule tomake the rubber vulcanizable with a sulfur vulcanization system.Epichlorohydrin homopolymer and copolymer rubbers are "cured"(cross-linked) with difunctional vulcanizing agents such as polyaminesand thioureas. Examples of suitable vulcanizing agents are hexamethylenediamine carbamate, hexamethylenetetramine, mixed polyamines,2-mercaptoimidazoline, ethylenethiourea, 1,3-diethylthiourea, piperazinehexahydrate and trimethylthiourea. Unsaturated epichlorohydrin rubbersmay also be vulcanized with sulfur cure systems suitable for other lowunsaturation diene type rubbers. Epichlorohydrin rubber is commerciallyavailable under the trademarks of Hydrin® Elastomers and Herchlor®Rubber. Epichlorohydrin homopolymer rubber is sold under the tradenamesof Hydrin 100 and Herchlor H. Epichlorohydrin copolymer rubber is soldunder the tradenames of Hydrin 200 and 270 and Herchlor C. Sulfurcurable epichlorohydrin copolymer rubber containing about 2%unsaturation is sold under the tradename of Hydrin 400.

Suitable thermoplastic polyesters comprise linear, crystalline, highmolecular weight solid polymers having recurring ##STR1## groups withinthe polymer chain. The term "linear" as used herein in respect topolyester means a polymer in which the recurring ester groups are withinthe polymer backbone and not pendant therefrom. Linear crystallinepolyesters having a softening point of 50° C. or above are satisfactorywith polyesters having a softening point or melting point above 100°being preferred with polyesters having a softening point or meltingpoint between 160°-260° C. being more preferred. Saturated linearpolyesters (free of olefinic unsaturation) are preferred, however,unsaturated polyesters may be used provided that the rubber iscross-linked prior to blending with the polyester or provided that therubber is dynamically cross-linked with a cross-linking agent that willnot significantly induce cross-link formation in the polyester.Cross-linked polyesters are unsatisfactory for the practice of theinvention. If significant cross-link formation of the polyester ispermitted to occur, the resulting composition is not thermoplastic. Manycommercially available thermoplastic linear crystalline polyesters maybe advantageously employed in the practice of the invention or they maybe prepared by polymerization of lactones, or one or more dicarboxylicacids, anhydrides or esters and one or more diols. Examples ofsatisfactory polyesters are described in the Coran et al patent, supra,particularly, Columns 7-8, the disclosure of which is incorporatedherein by reference.

The properties of the compositions of the invention may be modified byaddition of ingredients which are conventional in the compounding ofepichlorohydrin rubber and polyester. Examples of such ingredientsinclude carbon black, silica, titanium dioxide, pigments, clay, silanes,titanates or other coupling agents, stabilizers, antidegradants,plasticizers, processing aids, adhesives, tackifiers, wax, anddiscontinuous fibers such as wood cellulose or glass fibers, etc. Theaddition of particulate filler, preferably prior to dynamicvulcanization, is particularly recommended. Preferably, the particulatefiller is masterbatched with the epichlorohydrin rubber and themasterbatch is then mixed with the polyester. Particulate fillers suchas carbon black, silica, or silane-treated clay, improve the tensilestrength. Typical additions of particulate fillers or reinforcementfillers such as carbon black comprise about 20-150 parts by weight offiller per 100 parts by weight of rubber. The amount of particulatefiller which can be used depends, at least in part, upon the type offiller and the presence of other ingredients such as plasticizer.

Thermoplastic compositions of the invention are useful for making avariety of articles such as tires, hoses, belts, gaskets, moldings andmolded parts. They are particularly useful for making articles byextrusion, injection molding and compression molding techniques.Compositions of the invention also are useful for blending with otherthermoplastics, in particular, nylons and various polyester resins. Thecompositions of the invention are blended with thermoplastics usingconventional mixing equipment. The properties of the blend depend uponthe proportions. Generally, the amount is such that the blend containssufficient proportion of each component to obtain the desired effect.

Tensile properties of the compositions are determined by ASTM procedureD-638. Specimens are pulled with a tensile tester at 20 inches perminute to failure. The term "elastomeric" as used herein and in theclaims means a composition which possesses the property of forciblyretracting within ten minutes to less than 160% of its original lengthafter being stretched at room temperature to twice its unstressed lengthand held for ten minutes before release. True stress at break (TSB) isthe tensile strength at break multiplied by the extension ratio also atbreak, extension ratio being the length of a tensile test specimen atbreak divided by the original, unstressed length of the test specimen.Alternately, extension ratio is 1.00 plus 1/100 of the percent ultimateelongation. Especially preferred compositions of the invention arerubbery compositions having tension set values of about 50% or lesswhich compositions approximate the definition for rubber as defined byASTM Standards, Vol. 28, page 756 (D1566). More preferred compositionsare compositions having a Shore D hardness of 40 or below or a TSBbetween 30-100 MPa (megapascals) or a Young's modulus below about 100MPa.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Compositions illustrating the invention are prepared by chargingpolyester and epichlorohydrin rubber, in the indicated amounts (allparts by weight) to a Brabender mixer at about 185° C. and a mixingspeed of about 80 rpm. The rubber and polyester are mixed for asufficient time to melt the polyester and to obtain a uniform blend(about 2-3 minutes). Curative is added and mastication is continueduntil (generally between 2 and 6 minutes) maximum Brabender consistencyis reached. The composition is removed, cooled, and then returned to theBrabender mixer and mixed an additional 2-21/2 minutes. The material isthen sheeted and compression molded at about 225° C. Properties of themolded sheet are then measured and recorded.

Materials used to illustrate the invention are as follows: sulfurvulcanizable epichlorohydrin rubber comprising a copolymer ofepichlorohydrin, ethylene oxide and sufficient olefinic monomer to giveabout 2% residual olefinic unsaturation purchased as Hydrin® 400Elastomer. Polyester resin, a terpolymer of 1,4-butanediol,1,2-propanediol and terephthalic acid or ester, glass transitiontemperature 25° C., melting point 174° C., sp. gr. 1.25, yield tensilestrength 15.9 MPa, yield elongation 15%, tensile strength at break 15.9MPa, elongation at break 250%. Rubber curative consists of 3.34 parts byweight of zinc stearate, 2 parts by weight ofbis(2-benzothiazolyl)disulfide and 0.8 parts by weight of spider sulfurper 100 parts by weight of epichlorohydrin rubber. The effect of curingthe rubber and the effect of the polyester/rubber proportions are shownin Table 1.

                                      TABLE 1                                     __________________________________________________________________________                1   2   3   4   5   6   7   8   9                                 __________________________________________________________________________    Epichlorohydrin Rubber                                                                    90  80  70  60  50  40  30  20  10                                Polyester   10  20  30  40  50  60  70  80  90                                Curative    5.52                                                                              4.91                                                                              4.3 3.68                                                                              3.07                                                                              2.46                                                                              1.84                                                                              1.23                                                                              0.61                              Properties                                                                    TS, MPa     0.3 0.3 1.8 3.2 5.7 8.2 11.6                                                                              13.7                                                                              19.0                                          (4.6)                                                                             (6.4)                                                                             (7.8)                                                                             (9.4)                                                                             (9.8)                                                                             (11.7)                                                                            (13.1)                                                                            (13.8)                                                                            (13.7)                            M.sub.100, MPa                                                                            0.3 0.3 1.8 --  --  --  --  --  --                                            (0.6)                                                                             (1.5)                                                                             (3.0)                                                                             (3.9)                                                                             (6.5)                                                                             (8.3)                                                                             (10.4)                                                                            --  --                                E, MPa      0.3 0.9 5.3 21.3                                                                              42.4                                                                              68.4                                                                              113 144 173                                           (1.1)                                                                             (3.1)                                                                             (8.2)                                                                             (19.2)                                                                            (44.0)                                                                            (70.6)                                                                            (104)                                                                             (167)                                                                             (205)                             Elong., %   420 130 200 95  45  42  60  25  60                                            (740)                                                                             (580)                                                                             (540)                                                                             (690)                                                                             (640)                                                                             (580)                                                                             (580)                                                                             (59)                                                                              (90)                              TSB, MPa    1.8 0.8 5.4 6.2 8.3 11.6                                                                              18.6                                                                              17.1                                                                              30.4                                          (38.4)                                                                            (43.7)                                                                            (49.4)                                                                            (74.5)                                                                            (72.5)                                                                            (79.3)                                                                            (88.4)                                                                            (22.0)                                                                            (26.1)                            Tension Set, %                                                                            35  66  Bk  Bk  Bk  Bk  Bk  Bk  Bk                                            (5) (8) (15)                                                                              (25)                                                                              (43)                                                                              (49)                                                                              (58)                                                                              Bk  Bk                                __________________________________________________________________________

Control stocks are prepared without curatives. The properties of thecontrols are given without parentheses and the properties of thecompositions containing cured epichlorohydrin rubber are shown inparentheses. The data show that curing the rubber substantially improvesthe properties. For example, curing increases tensile strength in allcompositions containing 30 or more parts by weight rubber and increaseselongation in all compositions. The improvement in true stress at breakis greater in compositions containing a higher proportion of rubber. Incured compositions, extraction studies with methylene dichlorideindicate that the rubber is substantially cured. All compositions areprocessable as thermoplastics. Cured compositions containing 60 weightpercent or less of polyester are elastomeric.

Compositions consisting of different epichlorohydrin rubber anddifferent polyester are illustrated in Table 2. All parts are by weight.The epichlorohydrin rubber is a homopolymer of epichlorohydrin purchasedas Hydrin 100. The polyester of Stocks 1, 2, and 3 is the same as inTable 1. The polyester of Stocks 4 and 5 ispoly(tetramethyleneterephthalate), m.p. 225° C., designated in the tableas PTMT. The antidegradant is polymerized 2,2,4-trimethylquinoline.Stocks 1 and 4 are controls without curatives. The curative of Stock 2is a low molecular weight liquid polyamine, amine number 370-400purchased as Versamid 150. The curative of Stocks 3 and 5 consists of0.75 parts of ethylene thiourea and 2.5 parts of dibasic lead phosphate.Stocks 1-3 are prepared by the same procedure as Table 1. Stocks 4 and 5are prepared by a similar procedure except the mixer temperature isabout 225°-230° C. and sample specimens are molded at 250° C.

                  TABLE 2                                                         ______________________________________                                                       1    2      3      4     5                                     ______________________________________                                        Hydrin 100       50     50     50   50    50                                  Polyester (Table 1)                                                                            50     50     50   --    --                                  PTMT             --     --     --   50    50                                  Antidegradant    0.5    0.5    0.5  0.5    0.5                                Curative         --     2.5     3.25                                                                              --     3.25                               Properties                                                                    TS, MPa          6.6    13.5   16.1 10.2  26.3                                M.sub.100, MPa   6.6    5.9    6.6  --    18.2                                E, MPa           47     38     35   335   180                                 Elong., %        500    660    380  56    260                                 TSB, MPa         40     103    77   16    95                                  Tension Set, %   40     24.5   15   Broke 42.5                                ______________________________________                                    

All compositions are processable as thermoplastics. The data showsubstantial increases in tensile strength, elongation and true stress atbreak, TSB, resulting from curing the rubber. The data also show thatthe composition of Stock 5, comprising a blend ofpoly(tetramethyleneterephthalate) is stronger and that the compositionsof Stocks 2 and 3 comprising a blend of a terephthalate terpolymer ismore elastomeric (lower tension set).

Although the invention has been illustrated by typical examples, it isnot limited thereto. Changes and modifications of the examples of theinvention herein chosen for purposes of disclosure can be made which donot constitute departure from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A composition comprisinga blend of about 5-75 parts by weight of crystalline polyester, and,correspondingly, about 95-25 parts by weight of cured epichlorohydrinrubber.
 2. The composition of claim 1 which comprises about 10-65 partsby weight of polyester, and, correspondingly, about 90-35 parts byweight of epichlorohydrin rubber.
 3. The composition of claim 2 whichcomprises 40 or more parts by weight of epichlorohydrin rubber and whichis elastomeric.
 4. The composition of claim 3 in which the rubber is inthe form of discrete dispersed particles and the composition isprocessable as a thermoplastic.
 5. The composition of claim 4 in whichthe rubber is cured to the extent that no more than 20 weight percent ofthe rubber is extractable in a solvent in which uncured epichlorohydrinrubber is essentially completely soluble.
 6. The composition of claim 5in which the polyester has a softening point of 50° C. or above.
 7. Thecomposition of claim 6 in which the epichlorohydrin rubber is ahomopolymer of epichlorohydrin or a copolymer of epichlorohydrin andethylene oxide.
 8. An elastoplastic composition comprising a blend ofcrystalline polyester, in an amount sufficient to impartthermoplasticity to the composition, and cured epichlorohydrin rubber inthe form of dispersed particles of a size small enough to maintainthermoplasticity of the composition and which rubber is present in anamount sufficient to impart rubberlike elasticity to the composition. 9.The composition of claim 8 comprising about 20-60 parts by weight ofpolyester, and, correspondingly, about 80-40 parts by weight ofepichlorohydrin rubber.
 10. The composition of claim 9 in which thecured rubber particles are of a size of about 50 microns number averageor less.
 11. The composition of claim 10 in which the cured rubberparticle size is about 0.1 to 10 microns number average.
 12. Thecomposition of claim 11 in which the rubber is cross-linked to theextent that the cross-link density of the rubber is about 3×10⁻⁵ to3×10⁻⁴ moles per milliliter of rubber.
 13. The composition of claim 12in which the polyester has a softening point of 50° C. or above.
 14. Thecomposition of claim 13 in which the epichlorohydrin rubber is ahomopolymer of epichlorohydrin or a copolymer of epichlorohydrin andethylene oxide.
 15. The composition of claim 14 in which the rubber iscured with a polyamine cross-linking agent.
 16. The composition of claim14 in which the epichlorohydrin rubber is sulfur vulcanizable.
 17. Thecomposition of claim 16 in which the rubber is cured with sulfurcurative.
 18. The composition of claim 14 in which the rubber is ahomopolymer of epichlorohydrin.
 19. The composition of claim 14 in whichthe rubber is a copolymer of epichlorohydrin and ethylene oxide.
 20. Thecomposition of claim 18 in which the polyester ispoly(tetramethyleneterephthalate).
 21. The composition of claim 18 inwhich the polyester is a terpolymer of 1,4-butanediol, 1,2-propanedioland terephthalic acid or ester.
 22. The composition of claim 19 in whichthe polyester is a terpolymer of 1,4-butanediol, 1,2-propanediol andterephthatic acid or ester.